1. Field of the Invention
The present invention relates to hollow cylindrical motors.
2. Description of the Related Art
Up to now, some stepping motors having small diameters with respect to rotation axes and having increased outputs have been proposed (See Japanese Patent Laid-Open No. 09-331666 (U.S. Pat. No. 5,831,356)). FIG. 9 is an exploded perspective view illustrating component parts of a known stepping motor. FIG. 10 is a cross-sectional view illustrating the structure of the stepping motor after assembly. The stepping motor includes a rotor 201, a first coil 202, a second coil 203, a first stator 204, a second stator 205, an output shaft 206, and a connecting ring 207.
The rotor 201 is a permanent magnet divided into four sections in the circumferential direction that are alternately polarized. The first and second coils 202 and 203 are disposed adjacent to the respective ends of the rotor 201 in the axial direction. The first and second stators 204 and 205 are composed of a soft magnetic material, and are excited by the first and second coils 202 and 203, respectively.
The first stator 204 includes first outer magnetic poles 204A and 204B facing the outer periphery of the rotor 201 with a spacing therebetween, and first inner magnetic poles 204C and 204D facing the inner periphery of the rotor 201 with a spacing therebetween. The second stator 205 includes second outer magnetic poles 205A and 205B facing the outer periphery of the rotor 201 with a spacing therebetween, and second inner magnetic poles 205C and 205D facing the inner periphery of the rotor 201 with a spacing therebetween.
The output shaft 206 is fixed to the rotor 201, and supported by a bearing unit 204E of the first stator 204 and a bearing unit 205E of the second stator 205 so as to be rotatable. The connecting ring 207 is composed of a non-magnetic material, and retains the first and second stators 204 and 205 with a predetermined spacing therebetween.
In the stepping motor having the above-described structure, the rotor 201 is rotated by switching the current-carrying direction to the first coil 202 and the second coil 203 so as to switch the polarities of the first outer magnetic poles 204A and 204B, the first inner magnetic poles 204C and 204D, the second outer magnetic poles 205A and 205B, and the second inner magnetic poles 205C and 205D.
In this stepping motor, magnetic flux generated by energizing the coils flows from the outer magnetic poles to the opposing inner magnetic poles, or from the inner magnetic poles to the opposing outer magnetic poles so as to efficiently act on the magnet located between the outer magnetic poles and the inner magnetic poles. Moreover, since the distances between the outer magnetic poles and the inner magnetic poles can be reduced to approximately the thickness of the cylindrical magnet (rotor), the resistance of the magnetic circuit including the outer magnetic poles and the inner magnetic poles can be reduced. More magnetic flux can be generated with a small current, and the output can be increased as the resistance of the magnetic circuit is reduced.
The stepping motor of this type disclosed in Japanese Patent Laid-Open No. 09-331666 (U.S. Pat. No. 5,831,356) is cylindrical. When such a stepping motor for driving diaphragm blades, a shutter, a lens, and the like is disposed in a barrel of a camera so as to be parallel to the optical axis, the radius of the barrel is the sum of the radius of the lens, the radius of the aperture, and the diameter of the motor. Therefore, the diameter of the barrel of the camera cannot be sufficiently reduced. FIG. 11 illustrates the layout of a light-amount adjusting device, an aperture, and a motor shown in the axial direction. When diameters of a motor M, an aperture 301, and a barrel base plate (light-amount adjusting device) 300 are defined as D1, D2, and D3, respectively, the diameter D3 of the barrel base plate 300 is at least the sum of twice the diameter D1 and the diameter D2. Therefore, toroidal motors having thin thicknesses in the radial direction are demanded for such a use. Furthermore, a reduction in size of the barrel device or the light-amount adjusting device is also demanded.
A stepping motor that resolves the above-described problem is proposed in Japanese Patent Laid-Open No. 2002-51526. FIG. 12 is an exploded perspective view illustrating component parts of another known stepping motor. FIG. 13 is a cross-sectional view illustrating the structure of the stepping motor after assembly. The cross-sectional view is taken at the center of the axis. The stepping motor includes a rotor 401, a first coil 402, a second coil 404, a first stator 418, a second stator 419, and a connecting ring 420.
The rotor 401 is a permanent magnet divided into N sections in the circumferential direction that are alternately polarized. In this case, N equals 16. The first and second coils 402 and 404 are disposed adjacent to the respective ends of the rotor 401 in the axial direction. The first and second stators 418 and 419 are composed of a soft magnetic material, and are excited by the first and second coils 402 and 404, respectively. The connecting ring 420 is composed of a non-magnetic material, and coaxially retains the first and second stators 418 and 419 at a predetermined angle with a predetermined spacing therebetween.
In the stepping motor having the above-described structure, the rotor 401 is fitted into the connecting ring 420, and retained by bearing units 420a and 420i of the connecting ring 420 so as to be rotatable. Output for driving diaphragm blades and a lens barrel is extracted from pins 401t of the rotor 401.
Since the stepping motor is a hollow cylinder, the stepping motor can be disposed in a barrel of a camera so as to be parallel to the optical axis, and diaphragm blades, a shutter, a lens, and the like can be disposed inside the motor. Thus, the diameter of the barrel of the camera can be reduced.
However, in the stepping motor described in Japanese Patent Laid-Open No. 2002-51526, the magnet serving as the rotor is covered with yokes. Thus, the stepping motor needs the pins 401t in order to extract the output. This leads to increases in size, the number of parts, and costs.
Moreover, projections 420t of the connecting ring 420 extend inward in the radial direction from the outer periphery of the rotor 401. Therefore, the projections 420t must be elastically deformed when the rotor 401 is fitted into the connecting ring 420, resulting in poor assembling workability.
Furthermore, the axial-direction distance between the two bearing units 420a and 420i that support the rotor 401 such that the rotor 401 is rotatable cannot be longer than the length of the magnet (rotor) in the axial direction. Therefore, the rotor is easily inclined, and the rotation is not stable.
In addition, gaps in the radial direction are required between the rotor 401 and the stators. This leads to an increase in magnetic resistance and a reduction in space efficiency, and thus high torque cannot be obtained.